In an automatic assembly line or the like in the automobile industry, a clamping device for clamping a workpiece for a purpose of processing is used frequently. As such a clamping device, there are already known devices as disclosed in Japanese Patent Application Laid-open No. 2001-105332, Japanese Patent Application Laid-open No. 2001-310225, Japanese Patent Application Laid-open No. 2001-009741, and the like, for example.
In each of these clamping devices, a toggle linkage for transmitting a driving force from a driving source to a clamping arm is disposed between the driving source and the clamping arm to thereby rotate and move the clamping arm to a predetermined clamping position which has been set in advance and then generate a large clamping force for clamping.
In the above clamping device, a position where the workpiece is clamped by the clamping, i.e., a position where the clamping force is to be generated in the clamping arm has to be set in advance by adjusting the clamping device according to a size of the workpiece. In case of a midcourse change in the size of the workpiece, because it is impossible to immediately adapt to the change, the clamping device needs to be stopped temporarily to reset the clamping position. Moreover, when respective members forming the above toggle linkage and the like wear as a result of repetition of operation, the clamping position is displaced and the workpiece cannot be clamped accurately. Therefore, the clamping device needs to be readjusted periodically to reset the clamping position.
As described above, in the conventionally-known clamping device, the above-described troublesome setting operation of the clamping position is required so as to accurately clamp the workpiece in the clamping position by the clamping arm and therefore operation efficiency is decreased.